The present invention relates to golf balls having a clear finish or top coating and, in particular, to an improved polyurethane top coating composition for golf balls.
For decades, golf balls were finished by applying an opaque coating to the outer surface to the golf ball cover. To achieve a desirable white appearance, a multicoat paint system typically comprising a primer coat followed by one or more opaque coatings was applied to the golf ball. Such golf balls are typically referred to as "painted balls".
In the 1980's, a pigment was incorporated into the cover material prior to molding the cover of the golf ball, thereby eliminating the step of painting the golf ball. Such golf balls are typically referred to as "pigmented balls".
Whether the golf balls are painted or pigmented, identifying indicia such as the manufacturer's trademark or logo, or a model or identification number are stamped on the ball. In order to prevent the stamped indicia from being rubbed off, and also to impart a desirable glossiness to the ball, one or two clear finishes or top coatings are applied to the ball. Typically, such clear coatings consist of a clear epoxy primer, followed by a clear urethane top coat. The urethane top coat is usually a two part polyurethane, including a polyol part and a diisocyanate part. The two parts are mixed together and reacted to form the urethane coating.
While early urethane top coatings imparted a desirable glossiness to the ball, they suffered from certain disadvantages. For example, the urethane coatings made from aromatic diisocyanates tended to yellow very quickly upon exposure to ultraviolet light, thus detracting from the appearance of the painted or pigmented ball. Also, the early urethane coatings typically suffered from adhesion and abrasion resistance problems. Good adhesion is required to protect the indicia stamped on the ball, while good abrasion resistance is needed to maintain the glossiness of the ball.
To avoid the problems associated with the early urethane top coatings, urethane top coatings have been prepared using biurets and isocyanurate trimers of hexamethylene diisocyanate as crosslinking agents. Coatings made with such crosslinking agents exhibit superior adhesion and abrasion resistance, and superior color retention upon exposure to ultraviolet radiation. Top coatings using these crosslinking agents are disclosed in U.S. Pat. No. 5,459,220, which is incorporated herein by reference in its entirety.
Although the top coatings disclosed in U.S. Pat. No. 5,459,220 are qualitatively superior to earlier top coatings, application of these and other similar coatings to golf balls requires a drying or curing period. To describe the typical application process in more detail, the polyol and diisocyanate parts of the top coat are premixed in a pressure pot. Dry air is used to push the coating to an air atomizing spray gun where it is forced out of a small diameter fluid nozzle onto the surface of the golf balls. The sprayed golf balls are then placed into an oven for a predetermined length of time, and at a predetermined temperature, such that the solvent evaporates and the polyol and the diisocyanate polymerize to form the desired top coat. The oven times necessary to cure existing top coats can range anywhere from five minutes to six hours or more, while oven temperatures typically range from 140.degree. F. to 200.degree. F.
Regardless of whether the manufacturer uses a fast cure top coat (with cure times measured in minutes) or a slow cure top coat (with cure times measured in hours), the balls must spend some time in an oven to effect the cure. As the time necessary to cure a particular top coat increases, the golf balls must either spend more time in the ovens or be subjected to higher oven temperatures. Either way, greater amounts of energy must be applied to the golf balls to complete the manufacturing process, thus increasing the cost of manufacture. In addition, as a golf ball is exposed to greater amounts of heat, the possibility of damage to the surface of the ball itself is increased. For example, the surface of a modem golf ball is covered with spaced "dimples" to improve the aerodynamics of the ball when it travels through the air. Excessive heat exposure may cause the dimples to deform. Even a slight aberration in the shape of the dimples can adversely affect the ball's aerodynamic properties.